The invention entails the construction of a 3D model of a scene on the basis of a reference image possessing a depth map.
FIG. 1 represents the surface of an object in 3D space referenced 1. An approximation of this object by facetization or triangulation yields a model whose surface is referenced 2. A synthesized image is obtained by projecting the model onto a plane 4 from a viewpoint 6. The error 3 due to the approximation of the surface of the object to that of the model is conveyed on the image by a 2D distortion referenced 5. The figure shows that the error due to the approximation of the surface of a 3D object by planes may be conveyed by an error in the position of its silhouette in the image and more generally by distortion of the shape of the object and of the textures in the image.
A conventional type of 3D model is the representation by 3D facets. Known methods exist for the facetization of depth maps. One scheme entails an iterative method based on an initial triangulation of the depth map. This method is for example mentioned in the article by Xin CHEN and Francis SCHMITT entitled “Adaptive interval data approximation by constrained surface triangulation”, IFIP, Working conference on “Modelling in Computer Graphics”, pages 95–113, Genova, Italy, July 93.
FIG. 2 illustrates a “coarse to fine” type incremental facetization of this kind. More precisely, FIGS. 2a to 2d correspond to successive iterations, the first iteration, FIG. 2a, consisting in the creation of two triangles from the rectangular image. It should be noted that the facetization can be limited to a region of the image, previously polygonized and triangulated so as to constitute the initial model.
At each iteration, an approximation error map is calculated: this error calculated at all the points of the image corresponds to the difference between the depth value of the pixel and its approximate value through facetization. On the basis of this error map, the most poorly approximated point is detected: it is the one having the largest error in terms of absolute value. This error is compared with a threshold: if the error is below the threshold, the facetization scheme is terminated; otherwise, the corresponding point is introduced as new vertex of facets, and the facet to which it belongs is composed into three new facets, as is represented in FIG. 2b or else into four new facets if the point is situated on an edge, as represented in FIG. 2c for the next iteration.
In the known methods of facetization through the most poorly approximated point, a single threshold is used for all the points of the image. As a result, the quality obtained is not homogeneous. Moreover, the modelling cost, the information required in defining the model or representation cost may be considerable, for minimum required quality.
An aim of the invention is to elevate the aforesaid drawbacks.